The identification and recording of people via biometric features is becoming increasingly important. Alongside other biometric features, fingerprints play an important role. On the one hand, there are systems which are used to verify biometric features in which there must therefore be a match with stored features, for example in order to enable entry or access control. Other systems are used for identification by searching and storing in reference databases, for example in the case of border controls at airports or in the case of identity-recording by the police. For the latter systems there is a large number of requirements in terms of the quality, the resolution and the faithfulness to the original of the captured images of the skin textures. Not least because of the high demands of organizations entrusted with identity-recording measures, such as for example the Federal Bureau of Investigation (FBI), there is a high degree of standardization with these systems in order, on the one hand, to ensure as definite an identification as possible and, on the other hand, to make data sets which were captured by different systems comparable. For example, such systems must have a resolution of at least 500 ppi (points per inch), which corresponds to a pitch of the sensor elements of 50.8 μm. In addition, particular demands are made on the contrast transfer function (CTF), the signal-to-noise ratio (SNR) and the distortion. Finally, the grey scale must comprise at least 200 greyscale values and the image field must be illuminated as homogeneously as possible both in the immediate vicinity of the pixel and in the image as a whole.
All of the demand criteria require a balanced and high-quality system design. In the case of an optical system, this means, for example, that not only the acquisition sensor or acquisition sensors must satisfy the demands but also the illumination and all of the other components necessary for the image generation.
For recording finger- and handprints which fulfil the named high quality demands, at the present time, optical arrangements are predominantly used which operate according to the principle of disturbed total internal reflection. For this, a prism is applied, the surface of which provided for capturing the print must be larger than the surface required for capturing the print because of mechanical and optical demands. The size of the prism resulting from this often as a larger component in the capturing channel has a decisive influence on the minimum overall size and the minimum weight of a device.
On the other hand, however, the high image quality permits a rapid and reliable recording and identification of people, in particular even in the case of applications where, in addition to forensic accuracy, a high throughput of people also plays a role, for example in the case of border controls. In addition to the overall size and the weight, it is also disadvantageous that the use of complex mechanical components is necessary; moreover calibration and assembly are highly technically complex and time-consuming.
In order to combine the advantage of high image quality which can be achieved with disturbed total internal reflection with small, in particular flat, overall size, approaches are described, for example, in U.S. Pat. No. 9,245,167 B2. The fingerprint sensor disclosed there, in which the finger is placed on a TFT display (thin-film transistor display), captures a fingerprint and transmits this via an electronic system to a computer system. The brightness profile corresponding to the fingerprint forms—as in the case of arrangements with prisms—in that the epidermal ridges, the so-called papillary ridges, lying on the surface of the TFT display disturb the internal reflection of the light from the light source, while in the valleys between the papillary ridges, i.e. the epidermal valleys or papillary valleys, no contact occurs between skin and TFT display surface and there the light from the light source is reflected internally at the surface of the TFT display. A negative image of the fingerprint forms in this way on the light-sensitive areas of the TFT display. On the one hand, this solution assumes that the distance between the light-sensitive areas in the TFT display and the contact surface has a minimum size so that the light can strike the light-sensitive areas of the TFT. On the other hand, the illumination must fulfil certain requirements with regard to the direction of incidence and aperture angle.
U.S. Pat. No. 9,245,167 B2 discloses various possibilities as to how an illumination can be realized. One possibility consists of a light guide, arranged below the sensor layer, into which light is coupled from the side, which is coupled into the sensor layer from there. Because the device described there is designed for the examination of a single finger, the light guide can be kept relatively compact, with the result that a fall-off in the illuminance as the distance from the light source increases does not become disruptively noticeable. In the case of larger contact surfaces which are provided for the placement of several autopodia or a whole hand, however, the illuminance decreases with the result that the autopodia located at the edge are no longer correctly illuminated or their images are no longer completely usable due to a reduced contrast. In order also to obtain a sufficient illuminance for the finger placed on furthest from the light source, a powerful light source is required which, however, leads to the fingers which are closest to the light source being illuminated with excessive intensity, which also has an adverse effect on the contrast or leads to overexposure of these areas. An illumination which is optimal for these autopodia means, in contrast, that the autopodia which are placed on furthest from the light source are underexposed and can thus also not be represented. However, this type of illumination is not used with disturbed total internal reflection (TIR) in U.S. Pat. No. 9,245,167 B2; for such cases the use of a microprism array arranged between light guide and sensor layer is proposed when the light source is arranged below the sensor layer.
In other arrangements, the light is also coupled directly into a prism structure arranged above the sensor layer and above a cover glass. Another embodiment described in U.S. Pat. No. 9,245,167 B2 uses light sources which are arranged between the light-sensitive sensor elements; here, however, total internal reflection is not used for the image capture. This embodiment can also be used in conjunction with the scanning of a document.
When a microprism array is used, double images can form due to reflections in the light guide itself, furthermore parasitic scattered light from the surroundings can impair the image quality. In the case of the methods described in U.S. Pat. No. 9,245,167 B2, the protective layer on the sensor layer has to be kept as thin as possible since otherwise diffuse light generates optical crosstalk, which reduces the resolution. By this is meant the case where light from different angles of incidence strikes the same area of the contact surface and in the process records the same or very similar items of information about the finger placed on or not placed on at this specific point, but is also emitted at different angles of reflection due to the different angles of incidence and thus detected at various points, with the result that the resolution is impaired.
Further fingerprint scanners which operate according to the principle of disturbed total internal reflection with prisms, objective lenses and sensors are known in the state of the art, for example from U.S. Pat. No. 6,647,133 B1, from WO 2011/059496 A1 or from WO 2007/115589 A1. An optical fingerprint scanner with a touch-sensitive surface and an illumination via a light guide is described in WO 2008/033265 A2. Capacitive TFT sensors with a touch-sensitive control system are likewise known, for example from WO 2001/36905 A1 or from the already named U.S. Pat. No. 9,245,167 B2.
As already mentioned, prism devices are significantly larger and heavier than direct scanners, in particular when they are dimensioned for the simultaneous capture of several fingers. The calibration has to be effected very accurately, which is technically complex. Also ruled out is user guidance directly in the capture area of the skin prints. The capture of documents is also not possible using this principle.
Direct optical fingerprint scanners only work with a small distance between sensor element and skin surface, namely with distances of less than 25 μm. Although a robust encapsulation of the sensor/pixel structures can be achieved by means of such thin cover glass, the robustness against external mechanical influences is however reduced compared with thicker cover glasses. If an at least partially transparent coating is used instead of glass or ceramic, the stability vis-à-vis electrostatic discharges (ESD stability) is reduced compared with glass, and in addition such a protective layer only has a lower hardness and abrasion resistance than glass or ceramic. Such devices are therefore less suitable for scanning documents. Although glass with a thickness of less than 30 μm is commercially available and suitable for series production, it is however complex to process on large contact surfaces; in particular bubble-free optical bonding is challenging. Transparent ceramics in this small thickness are not available in series production. In principle the use of a thicker glass cover layer would be desirable, however this is accompanied by the associated reduced resolution described at the beginning.
Capacitive devices on the other hand cannot be used for the capture of documents; in addition the sensors are usually not transparent with the result that to date neither user guidance nor user feedback has been realized in the capture surface by a display mounted below it.